With the increasingly high integration level of the systems, the widespread use of the large power devices, the requirements of miniaturization of the product volume, more extensive application environments, and the continuous improvement of the system working speed, thermal management design has become an indispensable means to improve system reliability.
In the thermal design, considering the heat flux density of the device, the volumetric power density and temperature rise, the common cooling methods are natural cooling and forced air cooling. When the heat flux of the electronic device is more than 0.08 w/cm2 and the volumetric power density is over 0.18 w/cm3, the natural cooling alone cannot completely solve the cooling problem, and many systems require additional dynamic to maintain sufficient air flow to carry out forced air cooling or other cooling methods. The forced air cooling method which takes the fans as the main components are widely used.
The common fan temperature control schemes are: Pulse Width Modulation (PWM) intelligent temperature control fan, fan circuit serial thermistor, voltage comparator output to adjust the fan speed, and so on. In the PWM fan adjustment, depending on the temperature, the temperature control fan has a different speed regulator corresponding to the temperature, and since it is the real-time adjustment of the pulse width signal, the fan speed change is very sensitive, and the speed change and the temperature change are almost synchronous. The main principle of the fan circuit serial thermistor is that when the power supply starts to work, if the fan supply voltage is 7V, with the rising of temperature within the power supply, the resistance value of thermistor is gradually reduced, therefore, the fan voltage is gradually increased and the fan speed is also increased, so that the muting effect can be achieved when the load is light, and the cooling is ensured when the load is large. For the voltage comparator output to adjust the fan speed, an control electrical signal is generated by detecting the temperature and according to the temperature, the output end of the temperature measurement circuit is connected to the first input end of the comparator circuit, the second input end of the comparator circuit is input with the reference electrical signal, the comparator circuit compares the control electrical signal and the reference electrical signal, and generates an output electrical signal output to the speed control circuit so as to adjust the fan speed.
When it is required to be simply, fast, and low cost to achieve fan grading governing the aforementioned method becomes complicated and has some shortcomings at the same time. The PWM temperature control circuit takes a single-chip computer as the core, and the intelligent control of the fan can only be achieved through the close coordination between the hardware and software, which is relatively speaking, high cost, complicated implementation, and is generally applied to the scenes in which the fan speed is required to be precisely controlled. The drawback of the fan circuit serial thermistor method is that the low temperature range of the NTC thermistor is very wide, so what can be adjusted is only the fan speed but the stop of the fans can not be controlled when the temperature is suitable, thus it cannot meet the requirements of energy conservation. For the scheme of voltage comparator output to adjust the fan speed, it needs to design specialized temperature measurement circuit, comparator circuit and speed control circuit, of which the circuit design is relatively complicated.